Multilayered thermoforming processes typically involve providing a preformed multilayered sheet of thermoplastic material (usually on a roll), heating the preformed multilayered thermoplastic sheet to a thermoformable temperature, and contacting the heated multilayered thermoplastic sheet with a shaped mold surface. The heated multilayered thermoplastic sheet is typically drawn into contact with the shaped mold surface by means of a vacuum being drawn through perforations in the mold surface.
Such prior art methods of multilayered sheet thermoforming typically and undesirably involve multiple steps, such as separately forming the multilayered thermoplastic sheet, collecting the preformed multilayered thermoplastic sheet on a roll, transporting (e.g., shipping) the roll of preformed multilayered thermoplastic sheet to a molder (or fabricator), and re-heating the preformed multilayered thermoplastic sheet prior to the thermoforming operation.
Thermoforming processes that involve the continuous extrusion of a multilayered thermoplastic sheet, that is thermoformed using residual heat from the extruded thermoplastic sheet are known. U.S. Pat. No. 6,814,905 B1 discloses forming a thermoformable sheet having at least two distinct layers by means of co-extruders, and contacting the sheet with the mold surface of mold members (e.g., 300) provided on a rotating wheel (e.g., 50), while the sheet is in a substantially non-oriented state. Such continuous co-extrusion thermoforming methods typically involve the use of multiple extruders connected to a multilayer die. The use of multiple extruders is typically accompanied by substantially increased manufacturing costs. Increased manufacturing costs are generally due to duplication of equipment in that a separate extruder and the equipment associated therewith (e.g., raw material feed and temperature control apparatae) are required for each layer of the multilayered sheet. It can be difficult to control the thicknesses of the various layers of the co-extruded multilayered sheet, for example due to variable extrudate expansion upon emergence from the slots of the multilayered die. In addition, formation by co-extrusion of an outer layer having a width that is less than that of an abutting underlying layer typically and undesirably results in the outer layer having non-distinct edges that bleed out over the underlying layer, which can result in degraded physical and aesthetic properties of the final molded article.
It would be desirable to develop new multilayer thermoforming processes, and apparatae used therewith, that minimize or eliminate the steps typically encountered with prior art methods. In addition, it would be further desirable that such newly developed methods and apparatae also provide reduced operating costs, and/or improved control of the thicknesses and widths of the various sheet layers.